Dental surgeons currently use for taking impressions various types of materials. All those materials, at the moment of the injection have a pasty consistency. Example of such materials are:
the alginates, comprising powder alguae that the user mixes before using in a bowl with water for obtaining a pasty material that gels after some minutes;
the silicones, thiokols or elastomers which are viscous pastes that are mixed, before using, on a mixing plate or block with a catalysing or hardening paste or liquid. Those materials are polymerising after some minutes.
A problem with all those materials is the relatively short time--generally within 2 to 4 minutes--that is available for the user between the moment at which he starts the mixing and the moment where the material begins its polymerisation or gelification and is no longer injectable.
The prior art syringes for injecting those pasty impression materials are difficult to fill because the diameter of their internal bore is small.
A plurality of methods are used for filling classical syringes. First, generally, the syringes are filled by scraping their rear part on the mixture bowl for entering the material by gravitation. By this method, no suitable filling is obtained because it necessarily causes a mixture of air and material to enter the syringe, causing, during the injection, air bubbles to appear whereby impression quality is poor. Additionally, it is very difficult, within a short time duration, to scrape onto the whole surface of the mixing block the totality of the mixture, a part of the material is lost.
Syringes with charging systems integral with, or added to and taken away, their rear part are also known, those charging systems being filled with a spatula for taking the material from the mixing block or bowl. Those methods are not satisfactory because they cause time losses due to the handling of filling spare parts such as the piston of integral chargers or removable charger-piston sets. Those methods also cause a material loss and are often complicated to use and dirty the syringes.
Systems with double cylinder and double piston syringes are also known and can even be used for mixing some very fluid impression material. Those systems are not usable for pasty materials and are generally cumbersome and not suitable for injecting small quantities of materials.
It appears that those prior systems do not simultaneously present the following qualities: quickness of use, efficiency, material savings, cleanliness, and simplicity of use.